1. Field of the Invention
This invention relates to vapor phase epoxidation reactions of ethylene with molecular oxygen; and, more particularly to epoxidation reactions using specific activated silver catalyst.
2. Description of Prior Art
The preparation of ethylene oxide by the oxidation of ethylene in the presence of suitable catalysts is well known. These known processes can be generally separated into two groups; the first utilizes air and the second utilizes molecular oxygen, e.g., from about 85 mol percent to about 99 mol percent. "Silver catalysts" are utilized with both groups.
Although the first references to the use of silver as such a catalyst was made by Walter in British Pat. No. 21,941 (1905), it was not until some thirty years later that the first disclosures were made of the use of silver as a catalyst in the vapor phase oxidation of ethylene to ethylene oxide. See Societe Francaise de Catalyse Generalisee, French Pat. No. 729,952 (1932); and Lefort, U.S. Pat. No. 1,998,878 (1935).
Since silver is expensive, optimizing the amount of silver employed in a supported catalyst for a desired conversion and selectivity to products has been widely investigated. A variety of techniques have been developed fro the depositing of relatively small, but highly active amounts of silver on surfaces of non-silver supports such as alumina. For example, McKim and Cambron in Canadian Journal of Research, Volume 27, Section B (1949), pp. 813-827, describe a method for depositing particulate silver on a support by decomposing silver oxalate in aqueous ethanolamine at 60.degree. C. and forming a paste which is applied to the surface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962, to Endler, a silver coating formed by decomposition of a silver carbonate slurry is applied to a catalyst support surface.
Recently it has been disclosed that supported silver catalysts can be prepared by impregnating a porous substrate with certain silver containing solutions and evaporating or decomposing the solutions to deposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsen describes the use of an aqueous silver oxalate impregnating solution which employs a solubilizing/reducing agent of ethylenediamine, a mixture of ethylenediamine or ethanolamine and ammonia, or a mixture of ethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. No. 808,278 (1974) employs an aqueous solution of hexamethylenetetramine with an ethylenediamine silver complex to deposit silver on an alumina support by decomposition. Additionally, it has been disclosed in Japanese Pat. No. 71/19,606 to Fujii et al that impregnation of inorganic supports with aqueous silver nitrate/alkanolamine complexes with subsequent thermal decomposition gives supported silver catalysts for ethylene epoxidation.
It has now been discovered that an extremely stable, physically durable, supported silver catalyst used in accordance with commercially known processes such as described in U.S. Pat. No. 3,119,837, British Pat. Nos. 1,314,613 and 1,132,095, results in a process which gives higher unit productivity and selectivity to the desired product at lower operating temperatures. The selectivity is particularly important in air processes which are not closed systems because some proportion of the unreacted ethylene is lost by venting excess gas. The catalyst utilized in the inventive process can be simply produced by impregnating certain types of porous, inorganic substrates with a complex formed by dissolving a silver carboxylate in certains amines and thermally decomposing the complex to deposit the silver on the substrate and activate the silver. Additionally, the catalysts used in accordance with the instant process show high attrition resistance and surprisingly high mechanical strength.